I'm getting an issue where sometimes the video framerate suddenly drops to around a frame a second and stays there. The upload, render, and present queues drop to 1-2, with the decoder and subtitle queues still full. Audio's fine, and the video stays synced. It's not from the content being too complex, since once it starts it happens anywhere I jump in the video and it clears up if I close and reopen the video. It only affects some videos, but when it does it tends to keep coming back every few minutes of playback. Playback's normally perfectly smooth.
Right now with the problem active I have about 8GB of system RAM and 2GB of GPU RAM free. GPU load is only 9% during playback. Total CPU usage is about 60-70%, with 40-45% from MPC-BE and a GPU load of 9%. That's the best lead I have, since the same video playing smoothly only uses ~30% CPU but has a higher 18% GPU utilization. My CPU is 2 core / 4 thread, so it has to be using at least two threads to get those numbers, and if it's only using two threads then it might be hitting a CPU cap when it's having trouble.
Perhaps locking the affinity would help? Perhaps it doesn't have enough resources when both threads are either on the same core or on different cores? I don't know all the details of how SMP affects performance. If that were the case, though, I'm not sure why it would run fine for a while.
MPC-BE 64-bit 1.5.1 (build 2548)
Built-in audio and video decoders and audio renderer
LAV Splitter 0.70.2
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Both "don't analyze gradient angles for debanding" and "don't rerender frames when fade in/out is detected" were already checked. I didn't touch that section, so it should be set to defaults.
I don't think it's a problem with the quality settings being too high, since once the performance drops it's bad anywhere I jump, including parts that previously ran perfectly, and once I restart MPC-BE the passages where the trouble started now work fine. It seems to be more about how long it's been playing than the content of any given passage, since I'm frequently seeing it start in ordinary, low-motion scenes.
I'd try another player to see if it's the same.
Recently, MPC-HC running on my old XP PC decided to start dropping frames all over the place for 50fps and 60fps sources, even though they're standard definition.
I'm guessing it's something deep in the Windows internals, because nothing I tried made any difference. In the end I gave SMPlayer a spin. Exactly the same problem using the Direct3D output driver, but as soon as I switched to one of the drivers I don't quite understand yet (OpenGL, I think) the problem disappeared.
Years ago I had all sorts of video problems with a particular PC, especially when displaying subtitles. CPU usage would go mental and the video would stutter badly. I tried everything I could think of and then started trying anything else. The fix was to either disable over-clocking in the BIOS (it was only slightly over-clocked) or change the PCI-E clock in the BIOS from "Auto", to "100MHz". Computers....